Orifice coupling to resonant cavities



Oct. 11, 1955 w, EDSON ETAL 2,720,629

ORIFICE COUPLING TO RESONANT CAVITIES Filed Sept. 9, 1947 3 Sheets-Sheet 1 FIG. 7 E

as a/ a2 35 34 38 g 4 FIG. 8 4 4 3a v v bl a.

INVEN WA. 8V

Oct. 11, 1955 Filed Sept. 9, 1947 FIG. /8

ROTATAILE ATTENUATOR w. A. EDSON ETAL 2,720,629

ORIFICE COUPLING TO RESONANT CAVITIES 5. ii 1 I t l I 5* a 5 RESOMVT WAEDSON INVENTORS. R'WLANGE ATTORN Oc 1 1955 w. A. EDSON ET AL 2,720,629

ORIFICE COUPLING To RESONANT CAVITIES Filed Sept. 9, 1947 3 Sheets-Sheet 3 WAEDSON gj RWLANGE ATTORNE V United States Patent ORIFICE COUPLING TO RESONANT CAVITIES William A. Edson, Aflanta, Ga., and Russell W. Lange,

Chatham, N. 1., assignors to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application September 9, 1947, Serial No. 772,936

8 Claims. (Cl. 333-83) This invention relates to cavity resonators and more particularly to improvements in feeding Wave energy thereto.

An object of the invention is to enhance the concentration of the exicting electromagnetic field at the entrance to a resonant cavity.

Another object of the invention is to concentrate the electromagnetic field at the entrance to a resonant cavity in such a manner as to reduce intermode couplings.

A feature of the invention is a resonant cavity with a lineal feed slit in a wall thereof, at which the exciting electromagnetic field is concentrated by a tapered, impedance matching transformer.

Another feature of the invention is a wave energy supercharger feed device comprising a stepped quarterwavelength impedance matching transformer connected to a lineal slit in a resonant chamber.

Another feature of the invention is a wave energy supercharger feed device comprising a combination of a resonant wave guide transducer and/or a quarter wavelength step transformer for concentrating the exciting electromagnetic field at a lineal slit of a resonant chamber.

Referring to the figures of the drawing:

Fig. 1A shows the energy feed for a resonant chamber in accordance with the invention;

Fig. 1B shows the tuning piston end of the same resonant chamber;

Fig. 1C shows a schematic view of the passageway through the impedance step transformer;

Fig. 2A shows a side elevation of the tapered impedance matching transformer connected to a lineal slit in the wall of said chamber;

Fig. 2B is an end view thereof;

Fig. 3 shows a quarter wavelength step with the lineal slit impedance transformer associated therewith;

Figs. 4 and 5 show resonance transducers connected to the lineal slit;

Fig. 6 shows a combined resonance transducer and a quarter wavelength transformer feeding a lineal slit; and

Figs. 7 and 8 show directional couplers with concentrated energy feeds in accordance with the invention.

intermode coupling exists in a wave guide cavity or the like when there is energy transfer from one mode of oscillation to another. In high Q resonant cavities, such as echo boxes, intermode coupling results in a degradation of the desired performance characteristics of the operating mode.

Resonant chambers have heretofore been fed and excited at a circular or rectangular orifice located in the side or end wall thereof. The two types of orifice may give equal intermode coupling when adjusted for equal couplings to the main or operating mode, or the couplings may materially change in shifting the orifice'from the side to end wall or vice versa. Experimental Work indicates that the two also give equal performance as to polarization and that the rectangular slot produces less intermode coupling than does the equivalent round ori- 2,720,629 Patented Oct. 11, 1955 2 fice. The intermode coupling is primarily a property of the orifice and is apparently independent of the apparatus external to the resonant chamber echo box or the like.

Moreover, since the intermode coupling decreases with the size of the orifice, it becomes desirable to reduce the size and dimensions thereof to the smallest value consistent with existing mechanical limitations and tolerances of manufacture. Experimental work by applicants verifies the theoretical prediction that the loading on a cavity through a given slit orifice is substantially increased, when the characteristic impedance of the feeding guide is reduced by reducing its height.

Resonant chambers having a narrow feed slit in a Wall thereof, to which a tapered section of Wave guide is connected for improving the ringtime are disclosed in the United States application, Serial No. 772,987, filed concurrently herewith by W. F. Kannenberg. The present invention represents an improvement and extension of the invention disclosed in said application, particularly in narrowing the slit to achieve an appresiable reduction in intermode couplings and in concentrating the exciting field at the slit by means of impedance matching transformers and/or resonant transducers.

In accordance with the invention, resonant chambers, echo boxes or the like, are fed wave energy and excited by means of a supercharger, which may be defined as a narrowed slit in the wall thereof associated with an impedance matching transformer, resonant transducer or the like for increasing the concentration of the field thereat and reducing the intermode couplings in the chamber. The transformer may be a tapered impedance matching section, a

stepped wave guide section, or a resonant transducer, and suitable combinations thereof to further enhance the field concentration and correspondingly reduce the intermode couplings.

Referring to Figs. 1A and 1B, a cylindrical resonant chamber cut into two sections, the feed section 1 and tuning section 1, respectively, is shown, designed to operate in the TEUIn mode. The metallic end feed plate 2 is provided with a pair of mode suppressing slits 4, 4' spaced 180 degrees apart and at a distance approximately equal to /;R from the center C thereof to the slit midpoint, where R is the radius of the resonant chamber. The slits 4, 4' in the end plate 2 (diameter 2.04 inches) are 10 mils high, mils wide, when operating at 1.25 centimeter wavelength, and effectively coupled to the TEoin' mode without exciting the TEOZn mode, the slit 4 being the energy input feed for the resonant chamber. Impedance matching transformers 6 of the type, more fully disclosed subsequently, are connected to the chamber in such a manner as to match the impedance of the slits 4, 4' respectively at one end to wave guide feeds 8, 9 respectively, of rectangular cross-section at the other end. As shown in Fig. 1C, the input transformer 6 has a width of 420 mils, a height of 17 mils, and a length measured along the path of transmission of 152 mils. The output transformer (not shown) has a width of 420 mils, a height of 50 mils and a length of 152 mils, and is more loosely coupled to the cavity than is the input transformer. The output slit 4 is connected to a rectangular wave guide 9 for measuring the output energy from the chamber 1 by means of the crystal detector shown. The wave guides 8, 9 in an exemplary embodiment were each .42 inch wide x .170 inch high, the wave a 3 guide 8 serving as the input and the wave guide 9 as the output of the chamber 1.

The narrowed slits 4, 41 are preferably lineally diarmaeaa mensioned, i..e.,' reducedsubstantially to a.single; dimena s ion,- to-givev less intermode; coupling, than round. orifices or ordinary rectangular slots. The size. of, the slitsmay be, reduced, to an. extreme, limit and. still provide an improvedring-time in the resonant: chamber. by.using-,ther e with either. a tapered impedance.-matching section, or a quarter wavelength stepped. transformer or a resonance transducer as-shown in FigsaZA, 3; 4. V V V Themode suppressing. slits; 4-, 4' are also: disclosedin the United States applicationof R. W. Lange Ser-ial' No. 770,988, filed August 28, 1947, which issuedas United; States Patent 2,701,343, February l-, 1955.

The tuning pistonend 1' ofthe resonant chamber is describedinthe UnitedStates, application-of W. A. Edson, Serial-No. 776,194, filed September 26, 1947, which issued.asUnitedStates'Patent 2,710,945., June-14,1955.

Fig. 1C shows schematically the passageway construc tion, and dimensioning: of the .quarterrwave, form ofsupercharger represented in Fig.- 1A., InFig.; 1,(-l,,the input rectangular wave guide 85is connectedtto thequarter;

wavelength step transformer: 6, hollowed out; of: plate inthe, form. of a: rectangular'channel, and communicafi 7 ing with a lineal slit 4Icut into the silver facing}? of the endplat'e'.

resonator, and, the resulting magnification. of the, fields.

increases the couplingthroughllie; slit 4: 'HlepiSton 21 may be usedtoadjustzfon best resonance aboutthe.

position shown inEig. 4.

Fig. 5 shows a modified transducer arrangement for a resonance chamber or echo box. An inductive iris 26 having a-roundaperture 27 such as,- disclosed in the United Statesapplication:SeriaLNo..452,851,.filed July-30, 1942, by A. G. Fox, which issued as United States Patent 2,432,093, December. 9; 1947,. forms the endv wall of transducer. 20. A wave. guide. feed,line.28 is matched in Referring to Figs. 2A, 2B, an impedance; transformer.

6', is shown as a tapered'section of rectangular wave-guide connectedtoside wall 40 0f a resonantchamber at slit 4. The, rectangular feed pipe 8 is connected-integrally to one' end of the transformer 6 theflatter taperingdown therefrom in its height dimension b' tothe-dimensionsof thenarrow slit 4, over a length approximately-equal-to.

where n is an integer and Ag is the wavelength in the a guide. 7

Referring to-Fig; ,3, aquartenwavelength-steptransformer 10- corresponding to the transformer 6 in FigrlAg is shown for matching the impedance of the rectangular feed pipe 8 to the impedance of the'lineal-slit 4: The Y lengthof the transformer 10 is where x is the-wavelength in the'guide andits 'charac teristicimpedance Z1,= /Zz Z3 where .Z2-, represents.the impedance of the rectangularv guide. 8'andlZ the ,imp e,d;. ance at the slit 4. For a' rectangular,guide;.42 x170" forexample, Z2 may. have a typicallvalueofj 500.0hh1s.

' In practice, the quarter wavelength step itransformenlflis Thus, assumingqa ten-to-one ratio of wave guide heights at opposite ends of the transformer section 10, the equivalent coupling ratio is 100:1. This may be compensated for by a, reduction .ofthe. slit-lengthaby An advantage in intermode couplings of ten-'to-one and of Q dip at a crossing of one hundred-to-one shouldac company, the change.

Resonance transducers 20'in the supercharger feed as shown in Fig. 4 may be used' as impedance matching transformers. The transducer may be a: rectangular section ofgwave guide 7 in -length, connected to. a. cylindrical echo box or; 6. 0? nant chamber; side wall 40. for concentrating;the electro;

impedance by the iris 26 to the. impedance to the left thereof represented by thetransducerandslit in combination.

Fig, Gshowsaresonance transducer30 combined with a type of impedance matching; transformer 31 similar to. that:,shown.in Fig.3 3 forenhancing theconcentration of thee exciting-field. at; slit. 4. The combination of transducer 30 and step transtormer31j in the, supercharger feed providesa progressive reduction in impedance: going from the echo box or cavity toward the wave. guideline 28, namely, the transformer 31 accomplishing the first reduction and the transducer 30 the second reduction. The iris 26provides the efiect of, an inductancershuntedacrossthevwave guide feedline 28L v Thus, by way ofexample, the,value ofjiris 26 may be chosen so that the eifective series resistance of this basically shuntcombination, is 50 ohms The. length of transducer section 30Ihetween the iris-26 andthe quarter wavelength step 31f is then so chosei1 that'the impedance isa pureresistanceclosely equalto 5000,ohms., Thestep transformer 31, which is a quarterwavelength long, again inverts this impedance, about its own. value of 50 .ohms to the low valueof 05, ohm... For, opfimumringtimein this connectiomfthe.orificesize, mustbe reduced. so that the, cavity has an equivalent resistance of 0.05 ohm; 1

It is seen that the impedance level of the. cavity is lo ered. by; afactor, often witheach elaboration of the feed 1' device. Naturally this, factor. is arbitrary,.but it is illustrative of the relatively, large effects, which may he obtained. It is worth; noting that the length of slitjinFi'gAmus tbe.

electric lowers the cut-ofijtr'equency of a wave guide by the .factor; lz isathe, dielectric constant, Thecharacteristicflimp edancejs, likewise. lowered; but by asomewhat different; factor, It ,issthereforta Possiblev .to secure; a. re.- duction..in theirequiredi size- 0f ,both orificend feedin g guide by this expedient. The use of plastic dielectrics having relatively large values of dielectric constant may be desirable as a supplement to impedance transformers or resonant transducers, particularly in cavities where a relatively heavy loading is desired.

It should be understood that the resonant chamber or echo box may be drilled with a round orifice and be associated with an impedance transformer whose end constitutes a sort of slit orifice, which is set back from the inner surface of the chamber proper by a small circular recess. This recess tends to vanish as the thickness (measured along the direction of transmission) of the chamber wall approaches zero.

It should be understood that it is fully within the spirit of the invention to use dumbbell-shaped slits in conjunction with the impedance matching transformers and resonance transducers for further reducing intermode couplings in echo-boxes and the like.

Circular wave guide sections, elliptical or any other suitable wave guide shapes may be used for the impedance matching transformers and transducers heretofore described and illustrated as rectangular in cross-section.

It should be understood that although the impedance transformers and resonance transducers associated with a lineal slit to form a supercharger are frequently designed on the basis of single frequency operation, nevertheless their usefulness and operability has been extended over a band of frequencies approximately :6 per cent from the operating frequency.

Referring to Figs. 7 and 8, directional couplers are shown for wave guide transmission or the like, utilizing concentration of the electromagnetic field at the coupling slits for reducing the transmission loss therethrough. For details of general construction and mode of operation of directional couplers, reference is made to the United States application Serial No. 540,252, filed June 14, 1944, by W. W. Mumford, which issued as United States Patent 2,562,281, July 31, 1951.

The auxiliary wave guide 38, which may be rectangular in cross-section as shown, is coupled to the main guide 32 by narrow slits 34, 35, 36, 37 in staggered and overlapping relation as shown in Fig. 7. The centers of adjacent slits are spaced apart 4 where Ag is the wavelength in the main guide 32. The length of the slits may be suitably chosen in accordance with the couplings desired. For enhanced transmission, the length of the slits may be In prior art construction of directional couplers, where the coupling apertures were collinear, the transmission therethrough could be increased by increasing the size of the apertures. A limit to further increase the transmission resulted when the apertures merged.

In the stagger and overlap of apertures 34, 35, 36, 37 disclosed in Fig. 7, this physical limit to increase of transmission is obviated and directional couplers with low loss characteristics may thereby be produced.

The wave generator 31, load circuit 33, and coaxial lines 39 are as disclosed in the aforementioned application of W. W. Mumford.

Referring to Fig. 8, a modified low loss directional coupler is shown, wherein the heights of the auxiliary guide 38 and the coupling slits 44, 45, 46 are sufliciently reduced to provide an enhanced concentration of field thereat. The auxiliary wave guide section 38 and the corresponding portion of main guide 32 have a height b which is approximately A of the standard wave guide height. The standard rectangular wave guide may be characterized by two dimensions a= /2 kg The transition from wave guide 32 to the dimensions" of the main guide 33 may be effected by an impedance matching taper section as disclosed in the United States application of A. E. Bowen Serial No. 715,587, filed December 11, 1946, which issued as United States Patent 2,603,709, July 15, 1952.

What is claimed is:

1. In combination, a high Q cylindrical cavity resonator, a conductive circular end wall having a lineally dimensioned slit at where R is the radius of said end wall adapted to couple principally into an operating TEOln mode and being dimensioned to reduce the intermode coupling, said slit being located in a region of high field intensity for the operating mode and means for concentrating the electromagnetic field intensity at said slit, comprising an impedance matching transformer connected thereto, said transformer comprising a solid end plate whose thickness is comparable to a fraction of a wavelength, said plate and wall forming a seal against energy loss, a hollow channel in said end plate for transmitting electromagnetic waves above cut-off, the length of said channel in the direction of propagation being where Ag is the wavelength.

2. In combination, a cylindrical cavity resonator including a conductive disc forming an end wall thereof, said disc having a coupling aperture therein located at a position of maximum field strength for a desired TEomn mode, means for exciting an oscillating electromagnetic field therein having a desired mode of oscillation comprising a wave guide feed of uniform cross-section therefor and means for increasing the energy density at said aperture comprising an impedance matching transformer section connected between said feed and aperture, said transformer comprising a solid end plate whose thickness is comparable to a fraction of a wavelength, said plate and wall forming a seal against energy loss, a hollow channel in said end plate for transmitting electromagnetic waves above cut-off, the length of said channel in the direction of propagation being where Ag is the wavelength.

3. The structure of claim 2, wherein said impedance matching section comprises a solid end plate for said resonator having a lineal slit in one face thereof and a rectangular opening in the opposite face thereof connected together by a channel, the length of said channel being substantially a quarter wavelength and a thin silver facing for said end plate having a registering slit therein.

4. An echo box adapted for operation about a predetermined wavelength comprising a circular cylindrical cavity resonator having an internal diameter of several times said wavelength whereby said resonator is adapted to support electromagnetic waves of TEM and higher order modes, said resonator having a laminated end plate comprising an outer self supporting metal wall portion and an inner facing of thin metal of relatively higher conductivity than said supporting wall, said facing being many times thinner than said outer wall portion, means to excite waves of TED]. mode in said resonator compris ing a uniform hollow-pipe guide terminating at one end on said outer wall portion, said laminated end plate having an elongated aperture therein coupling the interiors of said resonator and guide, said aperture comprising a fine slit in said facing located at a point of maximum field intensity for said TEo1 mode and a rectangular passage in agmeaa.

said outer' wall portion otilesser size than guide, said fpassage'having alength' of substantially a quarter ofi' thewavelength of the waves transmitted theretlirough and a characteristic impedance that isthegeometric mean of the impedancepresented at said slit and the characteristic impedance of said guide, andasecond balancing slit in- 7 said facing similarly coupled to theTE-or mode;

55 Ina-combination, arectangulai' wave-- guide, an echo boxcavity resonatonand a circular flat end plate there between, said end plate having symmetrical-lineal slitstherein for exciting a TEOln mode in said resonator, said slits being located at a position of maximum field strength for the TEom mode and reduced in lineal dimension to minitnize'intermode coupling, said'' end: plate constituting animpedance step': transformer between said resonator and guide,;sai"cl= plate having ahollowrectangularchannel' tapering toward a -slit, the length ofsaid' channel being where-kg is' the' wave length in the-guide, said channel havinga charaeteristic'impedance for matching the wave guide and slit.

6. Incombination, a rectangularfeed" wave guide, a-

circular wave guide and an impedance step transformer in contact with said guides,-.said transformer. comprising a flat conductive plate having a lineal slit in one face thereof and a rectangular slit, in the opposite face adjoining said rectangular guide, and a rectangular channel cut in said plate in -lengtli-andhaving acharacteristic impedance for- V matching the impedancesof said feed guide and slit,

said slit being l'ocated at a-point of maximum field intensity fora desired 'FEbm-mode;

7; The-device of" claim and asecond2symmetficallydisposed -slit' locatedaatia point: d1? maximum-'rfieldintensity forsaid: des'i'red mode;

8: In combination;v a high: Q-. cylindrical echobox.

resonator having a conductive end wall, said? end well being-'- a circulatiplat'e having lineal 'slits at a position from the center; whewnws the radius of the end wall 7 References-Cited int'he' file'of' this patent UNITE STATES PATENTS 2,2 0t);02-3 Dallenbachi' May 7, 19.40.- 2 ,281,550 :Barrow May 5, 1942-. 231093913: 'Eonks':' ;n n n Oct; 22', 1946 2,421,912 Spoonert June; 10, 1947 2,432',093 Fox. Dec. 9, 1947 2,453,760 'Schellenga 2 Nov; 16,1948. 234553158- Bradley- NOV. 30, 1948 2;466,4'39 K'annenberg Apr. 5, 1949 2,576,186 Malter Nov. 27, 1951. 2591,695 Hansen Apr. 8, 1952 2666,869 Clo'gston Jan. 19; 19545 

